The invention relates to a process for coating carbon fibers with a refractory carbide; it also relates to the carbon fibers thus coated.
The manufacture of composite materials by reinforcing light metals such as aluminum, magnesium, titanium, or ceramic by using ceramic fibers is known in the art.
While the use of silicon carbide, boron or alumina fibers can yield excellent results from the standpoint of mechanical performance, nevertheless, the very high price of these fibers considerably limits their development.
The use of carbon fibers has heretofore been suggested. Unfortunately, the chemical compatibility of these fibers with a metal matrix is poor. In point of fact, when this type of composite is fabricated or employed, it has a troublesome tendency to undergo chemical reactions whereupon metal carbides are formed, which considerably degrade the properties of the composite.
In order to overcome these disadvantages, it has been proposed, therefore, to protect these carbon fibers with a coating which would then withstand the chemical action of the metal. It has thus been suggested to coat these fibers with a chemical deposit produced from a vapor phase of a refractory compound such as a titanium boride, nitride or carbide or a silicon or boron carbide.
This vapor phase deposition, referred to by the American acronym CVD (chemical vapor deposition), does not make it possible to obtain good protection of each of the elementary filaments of which the strand of carbon fibers consists. It is very difficult, if not impossible, in fact, to avoid preferential deposition, particularly on the peripheral regions of the strands, and also to avoid the welding of the filaments to each other. These disadvantages make this process unusable on an industrial scale.
In other words, the CVD technique, although theoretically highly attractive, does not make it possible to control the thickness and the homogeneity of the protective coating, especially when, as in a strand, the gaseous reaction medium does not diffuse well to the core. As a result, the individual carbon filaments are not coated homogeneously or uniformly to a controlled thickness.
In their communication, published in CVD V Conference Proceedings (1975) p. 623 to 633, Warren and Carlson described a process for coating titanium carbide onto carbon fibers by means of reactive CVD in a heated hermetic enclosure wherein a gas phase containing titanium chloride is used in the presence of hydrogen. This direct reaction, in which carbon is contributed by the fiber itself, can be written: ##STR1## Unfortunately, this theoretical process has seen no industrial follow-up, despite the long period which has elapsed since it became known and despite the increasingly pressing requirement and need to produce carbon fibers coated with a refractory carbide. It is firstly a noncontinuous process which takes a long time to implement resulting in very thick carbide deposits on the order of one micron. The mean diameter of the fibers is approximately seven microns. This makes these fibers brittle and, in most cases, the fibers are welded together. Briefly, this prior art process cannot be employed to coat a strand of parallel carbon filaments in a homogeneous, uniform and controlled manner, as required for the manufacture of composite materials containing light metal matrices.
In British Patent 1,235,011, corresponding to French Patent FR-A-2,028,338, it has been proposed to deposit amorphous boron continuously onto a carbon monofilament by passing a heated monofilament through a gaseous atmosphere of hydrogen and boron halide. Heating of the monofilament is achieved by a Joule effect provided by two mercury seals which are responsible for feeding current and for sealing the enclosure. This solution is limited to the deposition of boron and does not enable a strand of parallel filaments to be coated. The strand which constitutes a porous medium, passes through the mercury and the metal is drawn into the reactor on the strand and reacts with the gas mixture, thus degrading the seal.